An experimental investigation has been conducted to determine the local condensation heat transfer coefficient (HTC) of steam in the presence of air or helium flowing downward inside a 46-mm-i.d. vertical tube. The gas-steam mixture flow rate was measured with a calibrated vortex flowmeter before it entered the 2.54-m-long test condenser. Cooling water flow rate in an annulus around the tube was measured with a calibrated rotameter. Temperatures of the cooling water, the gas-steam mixture, and the tube inside and outside surfaces were measured at 0.3-m intervals in the test condenser. Inlet and exit pressures and temperatures of the gas-steam mixture and of the cooling water were also measured. The local heat flux was obtained from the slope of the coolant axial temperature profile and the coolant mass flow rate. For the air-stream experiments, the ranges of the test variables were as follows: mixture inlet temperatures of 100,120, and 140°C; inlet air mass fraction of 10 to 35%; and mixture inlet Reynolds number of ∼5000 to 22 700. For the helium-steam experiments, the ranges of the test variables were as follows: mixture inlet temperatures of 100, 120, and 140°C; inlet helium mass fraction of 2 to 10%; and mixture inlet Reynolds number of ∼5000 to 11400. The local HTC varied from 100 to ∼25 000 W/m2·°C. The local Nusselt number calculated from the obtained data was correlated in terms of the local mixture Reynolds number, Jakob number, Schmidt number, and gas mass fractions. It was found that for the same mass fraction of the noncondensable gas, compared with air, helium has a more inhibiting effect on the heat transfer, but for the same molar ratio, air was found to be more inhibiting.